The present invention relates to an error correcting method in an air flow measuring apparatus for measuring the air flow based on the radiation amount from the hot wire to the air using the hot wire, specifically to a measurement error correcting method for the measurement error caused by the intake air pulsation, especially by the reverse flow, when measuring the air flow to be taken into the internal combustion engine of the automotive vehicle.
As for the measurement error correcting method of the thermal resistor type air flow measuring apparatus in the pulsating operation area, a method using a correction map based on the throttle valve opening (xcex1) and the engine speed (N) as shown in Japanese Patent Application Laid-Open No. 8-105781 (1996) is generally known as prior art.
As for the method for reducing the detection error caused by the reverse flow by using the auxiliary air passage used in the thermal resistor type air flow measuring apparatus, Japanese Patent Publication No. 2-1518 (1990) is known. In this prior art, the influence of the reverse flow is reduced by means that a thermal resistor is placed in the auxiliary air passage having a bend, and that the reverse flow from the engine to the air cleaner is not at least directly applied to the thermal resistor.
In addition, as for the prior art most related to the present invention, Japanese Patent Publication No. 59-17371 (1984) is known. In this prior art, AC component of the output voltage signal from the thermal resistor type air flow measuring apparatus is extracted by the analog circuit and used for the error correction, and the negative error (so to say binary phenomena) in the detected air flow due to the non-linearity and the response delay of the thermal resistor is aimed to be increased.
It is difficult for the thermal resistor used as the air flow detecting device to detect directly the direction of the air flow due to its structural characteristic. For this reason, when the reverse flow occurs, the thermal resistor detects the reverse flow as the forward flow and this erroneous detection is reflected onto the detection error.
The air flow flowing into the intake air pipe of the engine is pulsated in responsive to the open and close of the intake air valve. The quantity of this pulsation is small when the throttle valve opens relatively with small amount, and increases as the throttle valve opens up to the full gate opening.
This phenomena is described by referring to FIG. 15. As the throttle valve is opened gradually while the engine speed maintained to be constant, in connection with the increase in the intake air flow velocity (air flow), the pulsation amplitude in the intake air pipe increases gradually, and the output of the thermal resistor indicates the value having a negative error due to its non-linearity and response delay when the pulsation amplitude reaches a certain large value. This phenomena is so-called binary phenomena. FIG. 13 shows a generation and development process of the binary phenomena. As the pulsation amplitude becomes much larger, the air flow in the intake air pipe contains a reverse flow. However, owing to the structural characteristic, it is difficult for the thermal resistor used as the air flow detecting device to detect directly the direction of the air flow, and hence, both the forward flow and the reverse flow are detected simply as a flow. Therefore, the thermal resistor detects the reverse flow, if any, as a flow, and consequently, a positive error is included in the detected signal.
For those reasons, in case of using the thermal resistor type air flow measuring apparatus, it is required to perform any correction operation when the reverse flow occurs.
It may not be simply concluded that the detection error due to the reverse flow is used for compensating only the reverse flow from the engine to the air cleaner side. This is because the reverse flow contributes again to the increase in the forward flow. FIG. 14 shows experimental results by the inventors. FIG. 14 shows the measurement result of the forward flow and the reverse flow in the air flow in the intake air pipe by using a special method. The engine is at the state that the reverse flow occurs, when the intake negative pressure reaches about 10 mmHg. In this case, in spite of measuring separately the forward flow and the reverse flow, the output signal for the forward flow also increases in the same manner as the reverse flow does. This is because the reverse flow contributes to the increase in the forward flow.
In order to prevent this phenomena, it is required to measure the forward flow component and the reverse flow component separately, and to subtract the reverse flow contribution from the forward flow component. However, it is required to make the response of the thermal resistor fast enough. In order to establish such a high response, it is required to make the size of the thermal resistor smaller by reducing its thickness in order to reduce the heat content of the thermal resistor. To make the size of the thermal resistor smaller leads not only to the reduction of the mechanical strength of the thermal resistor but also to the reduction of the anti taint damage and even the reduction of the output noise characteristic due to the higher response capability, and thus, the accuracy in calculating the mean air flow may be reduced.
In order to solve the above problems, a thermal resistor type air flow measuring apparatus of the present invention has an auxiliary air passage placed in an intake air passage of an internal combustion engine and having at least one or more bend, and a thermal resistor placed in said auxiliary air passage. The measuring apparatus further includes a judging means for judging an existence of a reverse flow in said intake air passage based on a maximum value and a minimum value of an air flow signal value obtained said thermal resistor.
By referring to FIG. 2, a concrete view of the means for solving the problem is described. FIG. 2 shows the relation between the air flow (AIRFLOW) on the vertical axis and the negative pressure (BOOST) in which the maximum value (Qmax) and the minimum value (Qmin) are measured from the detection waveform of the thermal resistor by changing the intake negative pressure (BOOST) by opening gradually the throttle valve while keeping the engine speed constant with which the negative pressure occurs. In FIG. 2, the pulsation amplitude (Qmaxxe2x88x92Qmin) and the mean value (Qmax+Qmin)/2, and the pulsation amplitude divided by the mean value ((Qmaxxe2x88x92Qmin)/(Qmax+Qmin)/2) are also shown. In FIG. 2, the reverse flow occurs when the negative pressure reaches about xe2x88x9210 mmHg and the positive error arises after this negative pressure value.
In the observation of the maximum value and the minimum value, in case that the mean value increases due to the reverse flow, it is proved that the change in the minimum value is small but the maximum value increases very much for the change in the intake negative pressure. In addition, in the observation of the pulsation amplitude divided by the mean value, its value increases as the pulsation amplitude increases. By defining a threshold value for the value of the division of the pulsation amplitude by the mean value, it can be judged that the reverse flow does not occur if the value of the division of the pulsation amplitude by the mean value is smaller than the threshold value, and that the reverse flow occurs if the value of the division of the pulsation amplitude by the mean value is larger than the threshold value.
And furthermore, in case that the reverse flow occurs, the larger the value of the division of the pulsation amplitude by the mean value, the larger the detection error. This relation is shown in FIG. 3. FIG. 3 includes the value of the division of the pulsation amplitude by the mean value, ((Qmaxxe2x88x92Qmin)/(Qmax+Qmin)/2), on the vertical axis and the error of the detected value to the actual value on the vertical axis. As the relation between the value of the division of the pulsation amplitude by the mean value and the error has a monotone increasing property, this relation and the maximum value and the minimum value of the detected waveform of the thermal resistor can estimates the existence of the reverse flow as well as the quantity of the reverse flow.
The reason why the layout of the thermal resistor in the auxiliary air passage having a bend is to prevent the deformation of the detected waveform of the thermal resistor due to the reverse flow. The result of the similar experiments to FIG. 2 in which the thermal resistor is shown in FIG. 9 and its output waveform is shown in FIG. 8. The major difference from the result shown in FIG. 2 is that the minimum value of the detected waveform increases as the reverse flow increases. This is because the thermal resistor detects straightforwardly the reverse flow and the waveform is folded and deformed extremely. In the area where the reverse flow occurs, the pulsation amplitude keeps a constant value independent of the change in the reverse flow, and the value of the division of the pulsation amplitude by the mean value decreases as the reverse flow increases. Thus, the monotone increase relation shown in FIG. 3 can not be obtained and it is difficult to judge the existence of the reverse flow and estimate the quantity of the reverse flow.
The reason why the detected value from the thermal resistor is converted from the output voltage to the air flow is to obtain the mean value of the detected value roughly from the maximum value and the minimum value. The output from the thermal resistor is such a non-linear output as its inclination is larger at the lower air flow and its inclination is smaller at the higher air flow, and even if the pulsation waveform approximately close to the sinusoidal waveform is detected as shown in FIG. 4, its output waveform has a sharp part at the lower air flow and a flat part at the higher air flow, and hence, it is difficult to estimate the actual mean value only from the maximum value and the minimum value.